Interim Standard IS-95-A (IS-95) has been adopted by the Telecommunications Industry Association for implementing CDMA in a cellular system. In a CDMA communication system, a mobile station (MS) communicates with any one or more of a plurality of base station subsystems (BSSs) dispersed in a geographic region. Each BSS continuously transmits a pilot channel signal having the same spreading code but with a different code phase offset. Phase offset allows the pilot signals to be distinguished from one another, which in turn allows the base stations to be distinguished. Hereinafter, a pilot signal of a BSS will be simply referred to as a pilot. The MS monitors the pilots and measures the received energy of the pilots.
IS-95 defines a number of states and channels for communications between the MS and the BS. For example, in the Mobile Station Control on the Traffic State, the BSS communicates with the MS over a Forward Traffic Channel in a forward link and the MS communicates with the BSS over a Reverse Traffic Channel in a reverse link. During a call, the MS must constantly monitor and maintain four sets of pilots. The four sets of pilots are collectively referred to as the Pilot Set and include an Active Set, a Candidate Set, a Neighbor Set, and a Remaining Set.
The Active Set are pilots associated with the Forward Traffic Channel assigned to the MS. The Candidate Set are pilots that are not currently in the Active Set but have been received by the MS with sufficient strength to indicate that an associated Forward Traffic Channel could be successfully demodulated. The Neighbor Set are pilots that are not currently in the Active Set or Candidate Set but are likely candidates for handoff. The Remaining Set are all possible pilots in the current system on the current CDMA frequency assignment, excluding the pilots in the Neighbor Set, the Candidate Set, and the Active Set.
Typcially, a BSS services a coverage area that is divided up into multiple sectors. In turn, each sector is serviced by one or more of multiple base transceiver stations (BTSs) included in the BSS. When the MS is serviced by a first BTS, the MS constantly searches pilot channels of neighboring BTSs for a pilot that is sufficiently stronger than a threshold value. The MS signals this event to the first, serving BTS using a Pilot Strength Measurement Message. As the MS moves from a first sector serviced by a first BTS to a second sector serviced by a second BTS, the communication system promotes certain pilots from the Candidate Set to the Active Set and from the Neighbor Set to the Candidate Set. The serving BTS notifies the MS of the promotions via a Handoff Direction Message. Then, when the MS commences communication with a new BTS that has been added to the Active Set before terminating communications with an old BTS, a “soft handoff” has occurred.
For the reverse link, typically each BTS in the Active Set independently demodulates and decodes each frame or packet received from the MS. It is then up to a switching center to arbitrate between each Active Set BTS's decoded frames. Such soft handoff operation has multiple advantages. Qualitatively, this feature improves and renders more reliable handoff between BTSs as a user moves from one sector to the adjacent one. Quantitatively soft-handoff improves the capacity/coverage in a CDMA system.
For example, FIG. 1 is a block diagram of communication system 100 of the prior art. Preferably, communication system 100 is a CDMA2000 or a WCDMA system. Communication system 100 includes multiple cells (seven shown), wherein each cell is divided into three sectors (a, b, and c). A base station subsystem (BSS) 101-107 located in each cell provides communications service to each mobile station located in that cell. Each BSS 101-107 includes multiple BTSs, which BTSs wirelessly interface with the mobile stations located in the sectors of the cell serviced by the BSS. Communication system 100 further includes a radio network controller (RNC) 110 coupled to each BSS and a gateway 112 coupled to the RNC. Gateway 112 provides an interface for communication system 100 with an external network such as a Public Switched Telephone Network (PSTN) or the Internet.
The quality of a communication link between an MS, such as MS 114, and the BSS servicing the MS, such as BSS 101, typically varies over time and movement by the MS. As a result, as the communication link between MS 114 and BSS 101 degrades, communication system 100 provides a soft handoff (SHO) procedure by which MS 114 can be handed off from a first communication link whose quality has degraded to another, higher quality communication link. For example, as depicted in FIG. 1, MS 114, which is serviced by a BTS servicing sector b of cell 1, is in a 3-way soft handoff with sector c of cell 3 and sector a of cell 4. The BTSs associated with the sectors concurrently servicing the MS, that is, the BTSs associated with sectors 1-b, 3-c, and 4-a, are known in the art as the Active Set of the MS.
Referring now to FIG. 2, a soft handoff procedure performed by communication system 100 is illustrated. FIG. 2 is a block diagram of a hierarchical structure of communication system 100. As depicted in FIG. 2, RNC 110 includes an ARQ function 210, a scheduler 212, and a soft handoff (SHO) function 214. FIG. 2 further depicts multiple BTSs 201-207, wherein each BTS provides a wireless interface between a corresponding BSS 101-107 and the MSs located in a sector serviced by the BSS.
When performing a soft handoff, each BTS 201, 203, 204 in the Active Set of the MS 114 receives a transmission from MS 114 over a reverse link of a respective communication channel 221, 223, 224. The Active Set BTSs 201, 203, and 204 are determined by SHO function 214. Upon receiving the transmission from MS 114, each Active Set BTS 201, 203, 204 demodulates and decodes the contents of a received radio frame. Each Active Set BTS 201, 203, 204 then conveys the demodulated and decoded radio frame to RNC 110, along with related frame quality information.
RNC 110 receives the demodulated and decoded radio frames along with related frame quality information from each BTS 201, 203, 204 in the Active Set and selects a best frame based on frame quality information. Scheduler 212 and ARQ function 210 of RNC 110 then generate control channel information that is distributed as identical pre-formatted radio frames to each BTS 201, 203, 204 in the Active Set. The Active Set BTSs 201, 203, 204 then simulcast the pre-formatted radio frames over the forward link. The control channel information is then used by MS 114 to determine what transmission rate to use.
In order to achieve higher throughput and lower latency, it is desirable to increase high data rate coverage of a reverse link. To achieve these requirements on the reverse link, communication systems such as communication system 100 have adapted techniques such as Hybrid Automatic Repeat ReQuest (H-ARQ) and Adaptive Modulation and Coding (AMC).
Adaptive Modulation and Coding (AMC) provides the flexibility to match the modulation and forward error correction (FEC) coding scheme to the average channel conditions for each user, or MS, serviced by the communication system. AMC promises a large increase in average data rate for users that have a favorable channel quality due to their proximity to a BTS or other geographical advantage. Enhanced GSM systems using AMC offer data rates as high as 384 kbps compared to 100 kbps without AMC. Likewise, 5 MHz CDMA systems can offer downlink and uplink peak data rates as high as 10 Mbps and 2 Mbps respectively through AMC, where 2 Mbps and 384 kbps was typical without AMC.
AMC has several drawbacks. AMC is sensitive to measurement error and delay. In order to select the appropriate modulation, the scheduler, such as scheduler 212, must be aware of the channel quality. Errors in the channel estimate will cause the scheduler to select the wrong data rate and either transmit at too high a power, wasting system capacity, or too low a power, raising the block error rate. Delay in reporting channel measurements also reduces the reliability of the channel quality estimate due to constantly varying mobile channel. To overcome measurement delay, a frequency of the channel measurement reports may be increased. However, an increase in measurement reports consumes system capacity that otherwise might be used to carry data.
Hybrid ARQ is an implicit link adaptation technique. Whereas, in AMC explicit C/I measurements or similar measurements are used to set the modulation and coding format, in H-ARQ, link layer acknowledgements are used for re-transmission decisions. Many techniques have been developed for implementing H-ARQ, such as Chase combining, Rate compatible Punctured Turbo codes, and Incremental Redundancy. Incremental Redundancy, or H-ARQ-type-II, is an implementation of the H-ARQ technique wherein instead of sending simple repeats of the entire coded packet, additional redundant information is incrementally transmitted if the decoding fails on the first attempt.
H-ARQ-type-III also belongs to the class of Incremental Redundancy ARQ schemes. However, with H-ARQ-type-III, each retransmission is self-decodable, which is not the case with H-ARQ-type II. Chase combining (also called H-ARQ-type-III with one redundancy version) involves the retransmission by the transmitter of the same coded data packet. The decoder at the receiver combines these multiple copies of the transmitted packet weighted by the received SNR. Diversity (time) gain as well as coding gain (for IR only) is thus obtained after each re-transmission. In H-ARQ-type-III with multiple redundancy, different puncture bits are used in each retransmission. The details for how to implement the various H-ARQ schemes are commonly known in the art and therefore are not discussed herein.
H-ARQ combined with AMC can greatly increase user throughputs, potentially doubling/trebling system capacity. In effect, Hybrid ARQ adapts to the channel by sending additional increments of redundancy, which increases the coding rate and effectively lowers the data rate to match the channel. Hybrid ARQ does not rely only on channel estimates but also relies on the errors signaled by the ARQ protocol. Currently, in CDMA2000 and WCDMA systems, the reverse link ARQ function, such as ARQ function 210, and a scheduling function, such as scheduling function 212, reside in an RNC, such as RNC 110. The location of the ARQ function and the scheduling function in the RNC is dictated by a need to support soft handoffs, since the soft handoff function, such as soft handoff function 214, also resides in the RNC. However, a result of locating these functions in the RNC is that the BSSs, such as BSSs 101-107 and their associated BTSs 201-107, can only communicate through the RNC, resulting in latency penalties.
Therefore, a need exists for a new architecture for a digital wireless communication system that will reduce the scheduling and ARQ delays of the prior art communication systems.